1. Field of the Invention
The invention relates to a linear motor, and more particularly, to a linear motor for positioning purpose.
2. Description of the Related Art
A linear motor is used in, e.g., a positioning apparatus, for actuating a table supported by a linear guide provided on a base.
A positioning apparatus shown in FIGS. 5A and 5B comprises a base 100, a induction-type linear motor LM, a pair of linear guides LG, and a table 101. The linear motor LM is fixed at the center of an upper surface of the base 100. The pair of linear guides LG are fixed on top of the base 100 along both sides of the linear motor LM and arranged in parallel with the linear motor LM. The table 101 is fixed on the linear motor LM and the pair of linear guides LG.
The linear motor LM comprises a stator 102 and a movable member 104. The stator 102 includes a fixing member 102a fixed at the center of an upper surface of the base 100 and a permanent magnet 103 fixed on an upper surface of the fixing member 102a. The movable member 104 is provided above the permanent magnet 103 and fixed on a lower surface of the table 101 by means of not-shown bolts (FIG. 5B). The movable member 104 has a plurality of straight magnetic poles provided at given intervals in the direction in which the movable member 104 is to move. Each of the magnetic poles is coated with an insulating material of predetermined thickness. A three-phase coil is wound around the thus-coated magnetic pole. Magnetic flux induced by the three-phase coil is perpendicular to the direction in which the permanent magnets 103 are arranged. An air gap of predetermined dimension is defined between the permanent magnet 103 and the movable member 104.
The linear guides LG comprise a pair of guide rails 105 and sliders 106. The guide rails 105 are fixed on an upper surface of the base 100 and at positions on both sides of the stator 102 and are arranged in parallel with the stator 102. The two sliders 106 are mounted on each guide rail 105. Therefore, a total of four sliders 106 are fixed on a lower surface of the table 101.
When an electric current is caused to flow through the three-phase coils of the movable member 104 of the linear motor LM, relative thrust acts between the movable member 104 and the stator 102 by means of interaction between magnetic flux induced by the plurality of straight magnetic poles of the movable member 104 due to the electric current and magnetic flux induced by the permanent magnet 103. As a result, the movable member 104 is moved along the guide rails 105. The table 101 is then moved while being guided by the guide rails 105. When an electric current is caused to flow through the three-phase coil of the movable member 104 in the opposite direction, drive force acts in opposite direction on the movable member 104, whereupon the movable member 104 is moved in the opposite direction. Thus, the movable member 104 and the table 101 can be moved to a predetermined position by appropriate selection of the direction in which the electric current flows through the three-phase coil of the movable member 104.
In related art, in order to increase the magnetic field induced by the magnetic pole of the movable member 104, the extremity of the magnetic pole is made thick, thereby increasing the area of the magnetic pole (FIG. 6A). In this case, the base portion of the magnetic pole around which the coil is wound becomes narrower, which poses inconvenience in winding a coil directly on the magnetic pole. Further, there is a necessity for winding a coil directly on the magnetic pole, thereby posing difficulty in winding the coil at high density. Further, a structurally inevitable gap arises between the magnetic pole and the coil, wherein the gap is not provided with a coil. For this reason, a limitation is imposed on the intensity of an obtainable magnetic flux.
In order to eliminate this inconvenience, as shown in FIG. 6B, a magnetic pole having the same thickness across its entirety; that is, from its extremity to the base portion (straight magnetic pole) and a former-wound coil fitted around the magnetic pole may be used. The former-wounded coil has been formed into the shape of a coil beforehand, wherein the coil has been wound at high density. In this case, an attempt can be made to minimize the gap between the magnetic pole and the coil, the gap being not provided with a coil. However, the quantity of magnetic flux induced by the magnetic pole is decreased correspondingly.
Further, when a plurality of magnetic poles of the movable member 104 are arranged at given intervals, every other magnetic pole is provided with a coil. The reason for this is that if all magnetic poles are provided with coils, the coils will come in close proximity to each other, thereby posing difficulty in assuring a gap between the adjacent coils.
However, a space for insulator to be used for coating the magnetic pole must be ensured in a related art coil space. For this reason, the extremity of the magnetic pole cannot be made thick correspondingly, thereby resulting; in a decrease in the quantity of magnetic flux induced by the magnetic pole.
Further, when the magnetic poles of the movable member 104 have a straight shape, the magnetic poles and the coils rub against each other when the coils are fitted around the respective magnetic poles, which is likely to cause an earth fault. Thus, the magnetic poles of straight shape are inferior in terms of ease of assembly.
When a plurality of magnetic poles of the movable member 104 are arranged at given intervals, only every other magnetic poles can be provided with a coil. Hence, the space for winding a coil cannot be utilized effectively.